The present invention relates to heterophasic copolymers, more particularly heterophasic copolymers of polypropylene, PP.
PP heterophasic copolymers, also referred to as PP block copolymers, comprise a polymer matrix with a dispersed rubbery copolymer phase. The matrix is a homopolymer or random copolymer matrix. The rubbery copolymer phase is a reactor blend of an amorphous rubber, a rubber-like polymer which is normally an ethylene-propylene copolymer (rubber), and a semicrystalline ethylene copolymer.
The heterophasic copolymers are produced in two or more reactors. The matrix homopolymer or random copolymer may be produced by standard polymerisation with Ziegler-Natta catalyst system in one or more slurry or bulk (loop) reactors or gas phase reactors or combinations of both. In a second stage, the polymerisation is continued and the rubbery copolymer phase is produced in the matrix polymer using one or more gas phase reactors.
The composition of the rubbery phase is controlled in the second stage by the ethylene/propylene ratio and the amount of hydrogen. The comonomer ratio (CR) ethylene/propylene in mol/mol, which is otherwise expressed as
CR=C2/C3
determines the rubbery copolymer composition. When CR is equal to or higher than 9, essentially ethylene copolymers are produced, and when CR is lower than around 0.1, essentially PP random copolymers are produced. Generally, the lower the CR, the less ethylene copolymers are present in the rubbery phase.
The amorphous rubber content is generally assessed by dissolving the polymer in xylene. The amount of xylene solubles, XS (weight-%) at room temperature (RT), corresponds to the amount of rubber. The rubber composition is defined by the ethylene content by weight-% in the xylene solubles, C2 of AM, where AM is the with acetone precipitated amorphous rubber content in the xylene soluble fraction, at RT.
The hydrogen in the second stage controls the molecular weight of the rubber, generally measured as the intrinsic viscosity (IV) of the with acetone precipitated amorphous rubber fraction (AM) of the xylene soluble fraction. Intrinsic viscosity is measured in decaline at 135xc2x0 C.
There is a continuing need for PP heterophasic copolymers with improved properties, notably materials with good flow and impact characteristics, especially for moulding, injection moulding, thin wall packaging and engineering applications.
PP heterophasic copolymers with a high melt flow rate, e.g. MFR greater than 40 g/10 min, and with medium or high impact strength are difficult to produce directly by polymerisation with a Ziegler-Natta catalyst system. The difficulty arises because the matrix polymer of a heterophasic copolymer needs to have an MFR which is 50 to 100% higher than the MFR of the final copolymer. In turn this requirement means that very high hydrogen concentrations have to be used in the polymerisation of the matrix polymer. In many cases that is not possible. In addition a matrix with a very high MFR is very brittle which then affects the whole copolymer.
Heterophasic copolymers with very high amounts of rubber (XS greater than 20 wt-%), so called super high impact copolymers or reactor-made thermoplastic olefin copolymers (rTPOs), are even more difficult to produce with high MFR.
Another known route to a high MFR product involves chemical treatment, i.e., visbreaking (peroxide treatment) of a molten PP heterophasic copolymer. The visbroken copolymer, also called a controlled rheology polymer, generally has low impact properties.
By visbreaking PP with heat or at more controlled conditions with organic peroxides the molar mass distribution, MWD, will be narrower because the long molecular chains are more easily broken up or scissored and the molar mass Mw will decrease, corresponding to an MFR increase. The MFR increases with increase in the amount of peroxide which is used.
Because the molar mass distribution is narrower and the molar mass is decreased by visbreaking, the flowability will be improved (=controlled rheology). The narrower MWD also changes the mechanical properties of a polymer. For example, visbroken PP homopolymers and random copolymers have lower stiffness (tensile modulus, flexural modulus) and slightly higher impact properties than a standard PP homopolymer or random copolymer with the same MFR.
During visbreaking, ethylene polymers and copolymers are cross-linked by the peroxide. That means that the molar mass of the ethylene polymer will increase corresponding to a great drop in MFR. This is seen as gel formation and lack of flowability.
Visbreaking of PP heterophasic copolymer, the block copolymer, is more complicated because of the complex blend composition of homopolymer or random copolymer matrix, amorphous rubber and semicrystalline ethylene copolymer. Both the matrix polymer and the rubber decrease in molar mass, giving higher MFR, but the ethylene copolymers are cross-linked and then will cause problems in the PP copolymer.
This negative reaction results in flow problems, or gels. The mechanical properties, both stiffness and impact, drop. The visbreaking of a heterophasic copolymer to a certain MFR consumes more organic peroxide than visbreaking of a corresponding homopolymer to the same MFR, because of the negative reactions with the ethylene copolymer. The consumption of the expensive peroxide increases the more rubbery copolymer there is in the heterophasic copolymer, and the more PE rich is the rubbery copolymer.
The present invention addresses the need for PP heterophasic copolymers with good flow and impact characteristics.
According to the present invention, there is provided a method of preparing a polypropylene heterophasic copolymer with good flow and impact properties, which involves visbreaking a polypropylene heterophasic copolymer with an optimised rubber composition. The amorphous rubber has an intrinsic viscosity of 2 dl/g or more and the ethylene content in the rubber, determined as C2 of AM is from 20 wt-% to 45 wt-%. The rubber should have a higher molecular mass than the matrix,
IVrubber greater than IVmatrix.
Thus, we have found that a heterophasic copolymer produced with an optimized rubber composition can be visbroken to high MFR materials without a significant loss of impact properties. The rubber needs to be produced with a low comonomer ratio and a high enough molecular weight.
Because the amorphous rubber decreases in molar mass on visbreaking, the starting heterophasic copolymer should be produced with a high enough rubber molar mass. The IV of AM should preferably be higher than 2 dl/g, more preferably higher than 2.5 dl/g so that the visbroken copolymers fulfill the criteria for a good impact copolymer.
Moreover, a suitably amorphous rubber is needed, which can be produced with CR greater than 0.25.
More particularly, good impact properties at low temperatures in a visbroken PP heterophasic copolymer are determined by the amount of rubber (XS and AM in wt-%) and the rubber copolymer composition. In particular, this invention involves visbreaking of PP heterophasic copolymer where the IV of the AMxe2x89xa72 dl/g and C2 of AM greater than 20 wt-% (corresponding to a CR greater than 0.25 mol/mol.), and the C2 of AM less than 40 wt-% (corresponding to a CR less than 0.7 mol/mol).
In this invention we define an optimized rubber composition of a heterophasic polymer for visbreaking to give good impact properties.
By xe2x80x9cstarting polymerxe2x80x9d is meant the heterophasic polypropylene polymer which comprises a polymer matrix with a dispersed rubbery phase. The matrix in the starting propylene polymer can be a homopolymer or a random copolymer. The rubbery copolymer phase is typically a reactor blend of an amorphous rubber, a rubber-like polymer which is normally an ethylene-propylene copolymer (rubber), and a semicrystalline ethylene copolymer.
The starting polymer is produced by any convenient route with which it is possible to prepare a polymer that fulfills the intrinsic viscosity and ethylene content criteria discussed below. Typically, the process comprises a first stage, in which the polymer matrix is prepared and a second stage, where the rubbery phase is produced in the polymer matrix.
The polymerisation is typically carried out in the presence of a Ziegler-Natta type catalyst. The Ziegler-Natta type catalyst typically used in the present invention is a propylene stereospecific, high yield Ziegler-Natta catalyst. The catalyst in the second polymerisation stage is typically the same that is used in the first polymerisation stage.
The first stage polymerisation can be carried out in one or more bulk reactor(s), preferably loop reactor, or in one or more gas phase reactor(s). Typically, the reactors are connected in series.
Comonomer(s) are optionally used in any or every reactor in the first polymerisation stage. Preferably as comonomers are used C2-C10 olefins, e.g. ethylene, 1-butene, 4-methyl-1-pentene, 1-hexene, dienes, or cyclic olefins, or a mixture thereof.
The second stage polymerisation, in which the rubbery phase is polymerised in the matrix, is preferably carried out in one or more gas phase reactor(s). The second stage polymerisation is typically carried out essentially immediately after the first stage polymerisation, i.e., the polymerisation product recovered from the first polymerisation stage is conducted to the first gas phase reactor in the second polymerisation stage. The gas phase reactors in the second polymerisation zone are preferably connected in series.
Ethylene is used as comonomer in any or every reactor of the second polymerisation stage.
Hydrogen can be used in different amounts as a molar mass modifier or regulator in any or every reactor in the first and second polymerisation stage.
According to a preferred embodiment, the heterophasic propylene copolymer is produced in a reactor system comprising at least one bulk reaction zone including at least one bulk reactor and at least one gas phase reaction zone including at least one gas phase reactor. The reactors are connected in series, the bulk reaction zone being preferably arranged before the gas phase reaction zone. A separation stage can be employed between the reaction zones to prevent the carryover of reactants from the first polymerisation stage into the second one.
According to this preferred embodiment the polymer matrix is produced in the first reaction zone, which is preferably a bulk reaction zone, and the rubbery phase is produced in the second reaction zone, which preferably is a gas phase reaction zone.
In addition to the actual polymerisation reactors used, the polymerisation reaction system can also include a number of additional reactors, such as prereactors. The prereactors include any reactor for preactivating and/or prepolymerising the catalyst with propylene and/or other xcex1-olefin(s) and/or ethylene, if necessary. All reactors in the reactor system are preferably arranged in series.
In summary, according to the preferred embodiment of the present invention, the polymerisation of the heterophasic propylene copolymer comprises at least the following steps of
subjecting propylene, and optionally comonomer(s) and/or hydrogen to polymerisation or copolymerisation in a first reaction zone or reactor in the presence of the catalyst,
transferring the first polymerisation product with the reaction medium into a second reaction zone or reactor, and feeding ethylene comonomer and optionally hydrogen and/or additional propylene into the second reaction zone or reactor,
continuing the polymerisation reaction in the second reaction zone in the presence of the first polymerisation product to produce a combined polymerisation product,
recovering the polymerisation product from second reaction zone.
To avoid the difficulties the starting heterophasic copolymer should be produced with such a rubbery copolymer fraction where the ethylene copolymer fraction is as small as possible. This can be done by appropriate choice of catalyst, and using low comonomer ratios, preferably CR is in the range of 0.25-0.7, more preferably in the range of 0.4-0.7 (mol/mol). This low comonomer ratio gives a lower amount of ethylene copolymer and then the visbreaking to higher MFR can be carried out with less peroxide.
The starting polymer before visbreaking has IV of AMxe2x89xa72 dl/g, more preferably 2.5-4.5 dl/g, and C2 of AM 20 wt %-40 wt %, which corresponds to a CR of 0.25-0.7. The MFR of the starting polymer is 0.1 to 60 g/10 min, usually more preferably 0.3 to 40 g/10 min. The amount of XS of the starting polymer is usually more than 10 wt-%, preferably 10 to 50 wt-%.
Furthermore, the intrinsic viscosity ratio of the rubbery phase to the matrix is suitablyxe2x89xa71, in other words, IVrubberxe2x89xa7IVmatrix.
The visbreaking is typically carried out essentially immediately after the production of the starting polymer, i.e., the product obtained from the second polymerisation stage can be conducted to visbreaking, which takes place in the pelletising stage in an extruder. It is also possible that the product of the second polymerisation stage is first pelletised, optionally stored, and the visbreaking is carried out using the pelletised product as starting material.
The visbreaking of the starting polymer, i.e., PP heterophasic copolymer to higher MFR can be carried out by standard methods with organic peroxides (e.g. Luperox 101 from Elf Atochem or Triganox 101 from Akzo) in an extruder at elevated temperature reflecting the so called half-life temperature of the chosen peroxide. Peroxides quantities of 150 to 1000 ppm are preferred, and the visbreaking is carried out at elevated temperatures of 180 to 300xc2x0 C., more preferably 200 to 250xc2x0 C.
The higher MFR of the product typically means MFR greater than 40 g/10 min for medium or high impact heterophasic copolymers. For super high impact copolymers with XS greater than 20 wt-%, e.g. reactor made grades (rTPOs), the higher MFR means greater than 10 g/10 min. More generally, the visbroken copolymer is normally produced with a visbreaking ratio, MFRfinal/MFRinitial, equal to or greater than 2, preferably at least 3 and often at least 4, which means that the MFR of the visbroken copolymer is at least 200% of the MFR of the starting polymer, preferably at least 300% and often at least 400% of the original value.
The impact properties of the visbroken polymer produced by this invention typically approximate to those of the polymer before visbreaking. In a preferred aspect, the impact properties of the visbroken polymer, notably the results for the instrument falling weight test at low temperature in accordance with IS06603-2, with the same kind of failure as the starting polymer, are 80%, more preferably 90% of the original value.
Next, the invention will be further illustrated with the aid of the following non-limiting examples.
MFR: The melt flow rate of the polymer material was determined according to ISO standard 1133 using a piston load of 2.16 kg and a temperature of 230xc2x0 C.
Xylene Solubles (XS):
Determination of xylene soluble fraction (XS):
2.0 g of polymer are dissolved in 250 ml p-xylene at 135xc2x0 C. under agitation. After 30xc2x12 minutes the solution is allowed to cool for 15 minutes at ambient temperature and then allowed to settle for 30 minutes at 25xc2x10.5xc2x0 C. The solution is filtered with filter paper into two 100 ml flasks.
The solution from the first 100 ml vessel is evaporated in nitrogen flow and the residue is dried under vacuum at 90xc2x0 C. until constant weight is reached.
XS%=(100xc3x97m1xv0)/(m0xv1)
m0=initial polymer amount (g)
m1=weight of residue (g)
v0=initial volume (ml)
v1 volume of analysed sample (ml)
Amorphous Rubber Fraction of the Xylene Solubles (AM):
Determination of AM:
The solution from the second 100 ml flask in the xylene solubles analysis is treated with 200 ml of acetone under vigorous stirring. The precipitate is filtered and dried in a vacuumoven at 90xc2x0 C.
AM%=(100xc3x97m2xc3x97v0)/(m0xc3x97v1)
m0=initial polymer amount (g)
m2=weight of precipitate (g)
v0=initial volume (ml)
v1 volume of analysed sample (ml)